Mr. Cantrell readily acknowledges concocting the crime. But what has drawn little scrutiny are his activities leading up to it. Thanks to important allies in Congress, he extracted nearly $350 million for projects the Pentagon did not want, wasting taxpayer money on what would become dead-end ventures.

Recent scandals involving former Representative Randy Cunningham, Republican of California, and the lobbyist Jack Abramoff, both now in prison, provided a glimpse into how special interests manipulate the federal government.